Flammability limits of stationary flames in tubes at low pressure

Knowledge of stationary premixed flames in small tubes close to the flammability limits is important in the development of small combustors. However, small scales hinder the experimental approach and enhance the system dependency of the flame. It was successfully demonstrated that the use of a large tube under low-pressure conditions (between 2 and 60 kPa) can be a model for atmospheric pressure flame in narrow tubes. Low-pressure limits of stationary flames depend on the equivalence ratio and can be extended to lower pressures by using larger tubes or by heating the tubes. Thus, the behavior and flammability limits of stationary flames were studied at low pressures. Stationary flames were classified into axisymmetric flames and nonaxisymmetric flames in terms of the flame shapes and flow rates. Propagation velocities of stationary flames at low pressures were found to be similar to laminar burning velocities at normal pressure. Near the flammability limits, the dead space between the flame and the wall was measured by directly imaging the flame. Flame thickness at the center was measured directly using the luminosity of a SiC filament. The quenching Peclet number of stationary flames based on the mass flow rate was found to be about 13, and the quenching Peclet number defined using the measured flame thickness was found to be about 11.5, regardless of pressure, equivalence ratio, tube diameter, and fuel employed (propane or methane). Close to the flammability limits, mass flow rate and flame thickness are significantly affected by the fluid mechanics and heat transfer process. Thus, a modified Peclet number that is based on the mass flow rate and the measured flame thicknesses is an important parameter that controls the quenching of a stationary flame.